Plasma etching is frequently used in semiconductor fabrication. In plasma etching, ions are accelerated by an electric field to etch exposed surfaces on a substrate. To effectively execute a plasma etching process, the power delivered from a radio frequency (RF) generator to the plasma chamber must be precisely controlled. An example of an RF generator for use in an RF plasma system may be found with respect to U.S. Pat. No. 6,707,255, issued on Mar. 16, 2004, the disclosure of which is incorporated by reference in its entirety herein.
A RF generator system typically includes a RF power source, a RF sensor, and a sensor signal processing unit. The RF power source generates RF power signals detected by the sensor. The sensor outputs analog sensor signals representative of the RF power signals. The analog sensor signals include a frequency of interest and a plurality of interfering frequency components. The sensor signal processing unit band limits the analog sensor signal to remove interfering frequency components from the analog sensor signal, thereby leaving an analog or digital representation of the frequency of interest.
It is desirable to accurately control the frequency and the phase of RF generators. For example, the typical phase offset for fixed or agile frequency of RF generators is not controlled. Phase values derived from a linear combination of the RF sensor signals also are typically not adequately controlled. Moreover, conventional RF generators do not externally control a variable phase offset, such as in an agile frequency RF auto tuning frequency system while maintaining a variable frequency offset between the RF generators.